1. Field of the Invention
The present invention relates to a nonreciprocal circuit device and, in particular, to a nonreciprocal circuit device such as an isolator or a circulator used in microwave bands.
2. Description of the Related Art
A nonreciprocal circuit device, such as an isolator or a circulator, has known characteristics that transmit a signal only in a predetermined direction and not in a reverse direction. Because of these characteristics, for example, the isolator is used in a transmitter circuit of a mobile communication device, such as an automobile telephone or a cellular phone, for example.
A 2-port isolator is described as a known nonreciprocal circuit device in International Publication No. 2007/046229. In the 2-port isolator, a first central electrode is wound by one turn around a rectangular parallelepiped ferrite and a second central electrode is wound by a plurality of turns around the ferrite such that the second central electrode crosses the first central electrode at a predetermined angle therebetween and is insulated from the first central electrode.
In the 2-port isolator, the first and second central electrodes are defined by an electrode layer on the surface of the ferrite. When the second central electrode is wound by a plurality of turns, a predetermined spacing S must be allowed with respect to line width L of the electrode layer. The line width L of the electrode layer cannot be increased. More specifically, if the line width of the second central electrode is decreased, resistance thereof increases, Q of the second central electrode decreases, and an insertion loss is increased accordingly.
For example, if a winding pitch is 100 μm, a spacing S of 30 μm is required, and the line width L has a maximum value of 70 μm. Due to the generation of insulation breakdown caused by a print smudge of the electrode layer, a solvent remnant in a photolithographic process, and diffusion into an insulator, the spacing S must be at least 30 μm. In other words, in the known 2-port type isolator including the electrode layer defining the second central electrode wound around the ferrite by a plurality of turns, the line width of the second central electrode is relatively small, and the improvement of the insertion loss of the 2-port type isolator is limited.
On the other hand, Japanese Unexamined Patent Application Publication No. 2006-157094 discloses a 2-port type isolator in which a ferrite is arranged in a horizontal position with a circuit board (with a main surface of the ferrite arranged in parallel with the surface of the circuit board). In the 2-port type isolator, a first central electrode and a second central electrode are arranged on the main surface of the ferrite so as to cross each other in an insulated state, and a line width of each central electrode on the portion thereof other than a crossing section is set to be different from a line width of the crossing section in order to adjust impedance of the isolator. In this 2-port type isolator, however, no consideration is given to decreasing the insertion loss by increasing Q of the second central electrode.